A newer method of washing yeast has lately been introduced by the invention of a specially constructed patented centerfuge. If it be intended to mix starch with the yeast, it is usually done just after the washing has been completed.
Potato or rice starch are used. The utmost carefulness must be observed in the examination of the starches, as they frequently are contaminated with bacteria or acids, which tend to injure the keeping qualities of the yeast and very soon become unfit for use.
After the clear water of the last washing has been removed by decantation, the compact settled mass is pressed dry by hydraulic or filter press, and finally formed by specially constructed machines into pound pieces, familiar to all bakers.
The “mash” or the “wort” after the yeast has been removed contain alcohol in paying quantities, and is recovered by distillation.
One hundred kilograms of mash yields an average of 11 per cent. yeast and 28 per cent. of alcohol, if fermented according to the “Vienna Process.” The “Aeration Process” yields 25 per cent. of yeast and 18 per cent. of alcohol. The remaining grains in the liquids are much sought after for their value as desirable fodder for cattle.
Yeast Adulteration.—High class compressed yeast should be free from all adulterants. Most manufacturers, on account of the slimy matter of yeast, causing many difficulties in pressing, add from 5 to 10 per cent. of potato starch, claiming that it increases the keeping qualities by absorbing part of the moisture. The writer, however, does not agree with them.
Starch is undoubtedly at times added to yeast in large excess; it then becomes an adulteration; this fraud is, however, readily detected by treating the sample of yeast with iodine. For this purpose break up a little of the yeast in a test tube with some water, shake it up well and add a few drops of tincture of iodine; after standing a little while the starch will settle at the bottom of the tube in a dark blue layer.
Plaster of paris has also been found in yeast; this, besides being fraudulent, is decidedly criminal, and verily, is giving a “stone for bread.”
Nature of and Examination of Compressed Yeast.—A good sample of compressed yeast should have a creamy white color. A brownish discoloration would indicate that fermentation had been too far prolonged before skimming. It should have an odor of apples, not cheesy; neither should it have an acid odor or taste. A piece of blue litmus paper pressed against the cut of the yeast should remain neutral or at the most show but a faint sign of red; a marked change in the paper from blue to red would indicate acidity.